1. Field of the Invention
The present invention relates generally to a communication system using a multiple access scheme, and in particular, to an apparatus and method for transmitting/receiving pilot signals in a communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
The development of mobile communication systems such as a 1x Enhanced Variable (1xEV) communication system and a High Speed Downlink Packet Access (HSDPA) communication system has widely contributed to opening the wireless multimedia service age. As a result, a subscriber station (SS) can access the Internet and receive a desired service even while on the move.
Research and the continuing development in a 4th generation (4G) mobile communication system is being made considering both software for developing various contents and hardware for developing a wireless access scheme having high spectrum efficiency to provide the best quality-of-service (QoS).
A description will now be made of the hardware considered in the 4G mobile communication system.
In wireless communications, a high-speed high-quality data service is generally affected by a channel environment. The channel environment in wireless communications frequently varies due to additive white Gaussian noise (AWGN); a change in power of a received signal caused by fading; shadowing; Doppler effects caused by movement of a subscriber station and a frequent change in velocity of the subscriber station; interference by other users; and multipath signals. Therefore, in order to provide a high-speed wireless packet data service, an advanced new scheme capable of adaptively coping with variations in a channel environment is required in addition to the scheme provided in the existing wireless communication system.
The typical wireless access scheme which has been partially introduced into a mobile communication system and is expected to be actively used for the 4G mobile communication system, includes such link adaptation schemes as an Adaptive Modulation and Coding (AMC) scheme, and a Hybrid Automatic Retransmission reQuest (HARQ) scheme.
The AMC scheme adaptively applies a modulation/demodulation scheme and a coding scheme according to fading on a wireless transmission line in order to maximally utilize capacity of the wireless transmission line. The HARQ scheme requests retransmission of received defective packet data in a physical layer to minimize a transmission delay, thereby improving QoS.
The use of the AMC scheme and HARQ scheme contribute to a remarkable improvement in the entire system performance. To use such a link adaptation scheme as the AMC scheme, a receiver must continuously measure a condition of a link between a transmitter and the receiver. In order for the receiver to measure the link condition, the transmitter must transmit a reference signal based on which the receiver can measure the link condition. A pilot signal is typically used as the reference signal.
Both the AMC scheme and HARQ scheme were proposed considering the link condition. That is, the AMC scheme and HARQ scheme are applied according to a measurement result on a pilot signal between the transmitter and the receiver. However, the 4G mobile communication system will actively perform data transmission through the uplink, and data transmission through the uplink also requires a link adaptation scheme that considers the link condition. Accordingly, there is a demand for a scheme of transmitting a reference signal using an uplink link adaptation scheme.